1. Field of the Invention
The present invention relates to a mobile communication system, a wireless base station, and a transmission rate allocation method, and particularly, to a transmission rate allocation method for allocating a transmission rate to a mobile station when the mobile station removes Uplink DPCCH (Dedicated Physical Control Channel) Gating to restart an HSUPA (High Speed Uplink Packet Access) communication in HSPA (High Speed Packet Access) evolution in a mobile communication system.
2. Description of the Related Art
In recent years, HSPA evolution has been examined in W-CDMA (Wideband-Code Division Multiple Access) as a way to speed up uplink packet communications or improve Latency.
A technique called Uplink DPCCH Gating is examined in the HSPA evolution.
The Uplink DPCCH Gating is a technique for terminating a transmission operation in an HSPA-related physical channel (UL DPCCH, UL=Uplink) and for using the physical channel only for data transmission in order to prevent uplink interference if no more data is to be transmitted to a wireless base station when a mobile station communicates with the wireless base station through the physical channel.
A practical use of the HSUPA is also examined (for example, 3GPP TS25.309) to improve the uplink coverage and throughput and to reduce the delay.
As in HSDPA (High Speed Downlink Packet Access), a technique, such as adaptive encoding and Hybrid ARQ (Automatic Repeat Request), is implemented in the HSUPA, and TTI (Transmit Time Interval) is 10 mTTI and 2 msTTI.
In the HSUPA, a wireless base station includes a MAC-e scheduler, and the MAC-e scheduler schedules an allowable transmission rate of a mobile station to make RTWP (Received Total Wide Band Power) below a predetermined level.
Other than the RTWP, the MAC-e scheduler schedules the allowable transmission rate of the mobile station based on the transmission rate of the mobile station (reception rate for the wireless base station), SI (Scheduling Information), the usage status in a wired zone, etc. The algorithm is not particularly defined.
An example of the operation of the MAC-e scheduler of the wireless base station will be described with reference to FIG. 1.
The wireless base station measures the RTWP at a certain cycle (step S11) and uses the measurement result for scheduling in the MAC-e scheduler.
The MAC-e scheduler of the wireless base station compares the RTWP measured in step S11 with an RTWP threshold (step S12).
If RTWP<RTWP threshold as a result of the comparison in step S12, the process moves to step S13. In this case, there is still an excess in electric power since the RTWP does not reach the RTWP threshold. Therefore, the transmission rate of one of the mobile stations can be increased. Thus, in step S13, the MAC-e scheduler selects a mobile station in which an increase in the transmission rate will be allowed. Although various algorithms can be considered, it is assumed here that the algorithm is for preferentially selecting a mobile station with a low transmission rate (reception rate in the wireless base station). The MAC-e scheduler then increases the allowable transmission rate of the mobile station selected in step S13 (step S14). However, even if RTWP<RTWP threshold, the allowable transmission rate is not increased in step S14 if the RTWP after the increase in the allowable transmission rate exceeds the RTWP threshold. Thus, it is appropriate that the MAC-e scheduler increases the allowable transmission rate based on a notification through E-AGCH (E-DCH Access Grant Channel, E-DCH=Enhanced-Dedicated Channel) only if the MAC-e scheduler determines that the allowable transmission rate can be increased.
On the other hand, if RTWP≧RTWP threshold as a result of the comparison in step S12, the process ends. In this case, the transmission in high electric power (high rate) cannot be allowed for the mobile station any more, because the RTWP is equal to or greater than the RTWP threshold. Under the condition (RTWP≧RTWP threshold), it is appropriate that the MAC-e scheduler decreases the allowable transmission rate of one of the mobile stations through the E-AGCH depending on the algorithm.
A system configuration and an operation, in which the wireless base station includes the MAC-e scheduler in the mobile communication system, will be described.
The system configuration will be described first with reference to FIGS. 2 and 3.
In FIG. 2, mobile station (#1) 10-1 and mobile station (#2) 10-2 are under the control of wireless base station 20 and are located in the same cell of wireless base station 20. Wireless base station 20 and mobile station (#1) 10-1 are in a state of HSUPA communication (hereinafter, “HSUPA communication state”) through a physical channel. Similarly, wireless base station 20 and mobile station (#2) 10-2 are in the HSUPA communication state. The same MAC-e schedulers included in wireless base station 20 schedule the allowable transmission rates of mobile station (#1) 10-1 and mobile station (#2) 10-2.
It is assumed that the state has changed from the state of FIG. 2 to the state of FIG. 3. In FIG. 3, since no more data is to be transmitted to wireless base station 20 in mobile station (#1) 10-1, wireless base station 20 and mobile station (#1) 10-1 are in an Uplink DPCCH Gating state (hereinafter “Gating state”) to prevent the uplink interference. Meanwhile, wireless base station 20 and mobile station (#2) 10-2 continue to be in the HSUPA communication state.
An operation, in which the mobile station removes the Gating state and restarts the HSUPA communication, will be described with reference to FIGS. 4 and 5. The system configuration corresponds to the configurations in FIGS. 2 and 3.
As shown in FIG. 2, it is assumed that mobile station (#1) 10-1 and mobile station (#2) 10-2 are under the control of wireless base station 20 and are in the HSUPA communication state. The relationship between the RTWP measured by wireless base station 20 and the RTWP threshold at this point is as shown in FIG. 5A. Other than the electric power of received signals from mobile station (#1) 10-1 and mobile station (#2) 10-2, the RTWP of wireless base station 20 includes electric power caused by interference from other cells and thermal noise. To avoid the RTWP from exceeding the limit value of noise rise, which is an increased amount of noise, the RTWP threshold is set below the limit value of the noise rise.
It is assumed that there is no more data for mobile station (#1) 10-1 to transmit to wireless base station 20 and that, as shown in FIG. 3, the Gating operation for terminating the transmission operation through the physical channel is started to prevent the uplink interference (step A1). Consequently, since mobile station (#1) 10-1 terminates the uplink data transmission, the transmission rate of mobile station (#1) 10-1 (reception rate of wireless base station 20) decreases (step A2). An average value, not an instantaneous value, is used for the transmission (reception) rate in many cases.
Since the transmission rate of mobile station (#1) 10-1 (reception rate of wireless base station 20) decreases, the MAC-e scheduler of wireless base station 20 decreases the allowable transmission rate of mobile station (#1) 10-1 through the E-AGCH (step A3).
As the transmission rate of mobile station (#1) 10-1 (reception rate of wireless base station 20) decreases the RTWP of wireless base station 20 decreases, and the difference between the RTWP and the RTWP threshold increases (step A4). The relationship between the RTWP and the RTWP threshold of wireless base station 20 at this point is as shown in FIG. 5B, and there is a vacancy by the amount of the transmission rate allowed for mobile station (#1) 10-1.
Therefore, the MAC-e scheduler of wireless base station 20 notifies, through the E-AGCH, the increase in the allowable transmission rate to a mobile station having the lowest reception (transmission) rate among the mobile stations other than mobile station (#1) 10-1 under the control [mobile station (#2) 10-2 in this case because the target mobile station is only mobile station (#2) 10-2, step A5]. In response, mobile station (#2) 10-2 increases the transmission rate (step A6).
In wireless base station 20, the reception rate of mobile station (#2) 10-2 increases due to the increase in the transmission rate of mobile station (#2) 10-2 (step A7). The RTWP also increases, and the difference between the RTWP and the RTWP threshold decreases (step A8). As a result, the transmission rate equivalent to the difference between the RTWP, which is generated by the start of the Gating operation by mobile station (#1) 10-1, and the RTWP threshold is allocated to mobile station (#2) 10-2. The relationship between the RTWP of wireless base station 20 and the RTWP threshold at this point is as shown in FIG. 5C.
It is assumed here that mobile station (#1) 10-1 has removed the Gating state to restart HSUPA communication (step A9).
However, the transmission rate of mobile station (#1) 10-1 at this point is still decreased because the allowable transmission rate is decreased in step A3. Therefore, mobile station (#1) 10-1 cannot perform the HSUPA communication at the transmission rate before the Gating operation has been started.
Even if the MAC-e scheduler of wireless base station 20 attempts to increase the transmission rate of mobile station (#1) 10-1, there is no difference between the RTWP and the RTWP threshold because the allowable transmission rate of mobile station (#2) 10-2 is increased in step A5. Therefore, the MAC-e scheduler of wireless base station 20 cannot instinct mobile station (#1) 10-1 to increase the allowable transmission rate.
As described, in the conventional mobile communication system, if a mobile station starts a Gating operation, the transmission rate allowed for the mobile station is allocated to another mobile station. Therefore, there is a problem in which the transmission rate before the Gating operation is started cannot be quickly restored when the mobile station in the Gating state restarts the HSUPA communication.